1. Field of the Invention
The invention relates to airbag flap hinges, in particular to airbag flap hinges comprising textile inserts sheathed with plastics material, and also to methods for the manufacture thereof.
2. Description of the Related Art
Airbags are used widely, in particular in motor vehicles, to protect vehicle occupants from damage caused by impact from various sides. Thus, it is currently conventional to provide airbags of this type not only in the steering wheel and also the instrument panel in front of the front passenger seat, but also in the door linings, optionally in the roof interior, at the sides of seats or the like. Increasingly stringent demands placed on the appearance of inner lining parts of this type require the airbag, and in particular the total airbag module, consisting of the collapsed airbag, the firing module, if appropriate a firing channel and the required support structures, to be arranged out of sight, within the lining parts and below at least the top decorative layer.
Precisely at those locations where the airbag is bulky and takes up a comparatively large overall space and also at those locations where the airbag is intended, when triggered, to extend over a large area, for example in the region in front of the front passenger seat within the continuation of the instrument panel, it has become established practice to insert what are known as airbag flaps above the airbag within the decorative surface. The airbag flaps perform a plurality of tasks, examples of which include supporting the decorative material above the collapsed airbag and protecting the collapsed airbag from mechanical damage.
In addition, these airbag flaps can, in the event of triggering of the airbag, positively assist the displacement of the decorative materials of the inner lining part in order to assist unfolding of the airbag as freely as possible in the desired direction. In the event of triggering, the decorative material preferably tears along defined tearing lines and the airbag flap preferably folds to a predefined extent about a defined folding axis and clears the path for the airbag as it unfolds.
It was quickly recognized in this regard that particular care must be taken when using flaps of this type to ensure that the flaps neither injure the vehicle occupant nor cause extensive damage in the passenger compartment. Thus, for example, on folding of the airbag in the door armrest region or in the instrument panel, it should be ensured that the airbag flap does not flap down out of the rest position so far that it strikes and possibly damages the windscreen or side window of the vehicle. It must also be ensured that the airbag flaps do not, in the event of triggering of the airbag, become completely detached from the environment and possibly fly unrestrained through the passenger compartment.
Sheet metal hinges, which are arranged between a hinge-side edge of the airbag flap and the surrounding region, are therefore usually inserted in the region of the instrument panel and door linings. These surrounding regions can, for example, consist of a specific frame structure made of plastics material or metal. Conventionally, the sheet metal hinges are joined to the carrier, in the final assembly of, for example, the instrument panel, by joining methods such as hot riveting, ultrasonic welding or using hollow and pop rivets or screwed connections.
Sheet metal hinges of this type are, however, comparatively complex to manufacture, require complex and multistep operations for manufacture and connection and, moreover, conventionally break when subjected to excessive material stresses so as to form sharp edges, thus again significantly increasing the risk of injury to a vehicle occupant in the event of a component failing. In addition, hinges of this type have comparatively high masses, as a result of which energy is consumed during folding-open of the airbag flap, leading either to delayed unfolding of the airbag or to the need for larger dimensions of the propelling charge in the firing module. Both alternatives are however undesirable.
In order to produce a feel and appearance which remains uniform beyond the site of installation, airbag systems of this type are configured in the decorative carrier so as to be integral with the airbag flap and the frame which may surround the flap. An example of this may be inferred from DE 4 437 773 C1.
On account of the drawbacks of sheet metal hinges, other approaches include woven fabric inserts sheathed with plastics material. The sheathing with plastics material allows an intimate and form-fitting bond to the plastics material of the airbag flap and also to the plastics material of the surrounding regions to be achieved. Woven fabric inserts of this type are known, for example, from DE 199 35 625 A1, DE 10 2004 014 423 A1 and EP 1 062 128 B1.
Woven fabrics of this type, which consist in a manner known to a person skilled in the art of interwoven warp and weft yarns arranged substantially perpendicularly to one another, are conventionally arranged in such a way that either the warp yarns or the weft yarns are arranged perpendicularly to the hinge-side edge of the airbag flap. On account of the fact that the forces acting on the woven fabric when the airbag flap is folded down also occur substantially perpendicularly to the hinge-side edge, these woven fabric inserts act substantially just on one type of the woven fabric yarns.
Woven fabrics of this type are described, for example in DE 20206379 U1 and DE 199 35 625 A1, as what are known as “woven fabric nets” which usually contain a plain or leno weave known to a person skilled in the art. These nets, which contain no knotted connection between the warp and weft yarns, allow comparatively simple extraction of individual yarns from the woven fabric. In use as a hinge, this entails the risk of component failure. Use has therefore also been made of a woven fabric in which leno weaves and size application provide a slightly better connection between warp and weft yarns than in the case of an uncoated plain weave.
Tearing of the yarns in the edge region or extraction of individual yarns from the plastics material is nevertheless possible at comparatively low forces below 1 kN.
In addition, force is transmitted between the plastics material parts and the conventional net hinge directly, in a single stage and without further damping properties. This means that in the most favorable case, in which the hinge does not become torn on folding-down of the airbag flap, the hinge absorbs the forces occurring up to its yield point, and above the yield point, diverts the acting forces wholly and without damping into the plastics material of the airbag flap and the surrounding regions.
The above-mentioned DE 202 06 379 U1 therefore describes a reinforcing woven fabric which is inserted at a 45° angle. Practical tests have revealed that on use of a reinforcing woven fabric of this type, an effect is achieved only by those yarns projecting beyond the portion of the hinge region by a distance of at least 20 mm on both sides. This projecting is required as otherwise it is not possible to achieve sufficient securing and thus the transmission of force from the net to the plastics material of the airbag flap and the surrounding regions. Damping of the energy is in this case caused by stretching of the net and slipping of the yarns in the plastics material and thus limits the force transmission of the hinge as a function of the adhesive forces between the yarns of the net and the plastics material surrounding the net. However, this has a direct influence on the security of the flap function and thus on passenger safety.
If, during folding-down of the airbag flap on triggering of the airbag, the maximum admissible tensile force acting on the hinge, and thus on the yarns thereof, is exceeded, the yarns start to slip in the plastics material, wherein conventionally only a very low effect of retaining the yarns in the plastics material may be observed. As a result of this extracting of the yarns, the hinge is again also extended beyond a desired degree, as a result of which the airbag flap, for example in an instrument panel, eventually moves dangerously close to the windscreen. There is also the risk of the yarns becoming completely extracted, leading ultimately to detachment of the airbag flap.
In the case of particularly thick decorative surfaces with, for example, a foam or spacer knitted fabric layer arranged between the decorative carrier and the decorative material, a textile hinge of this type should be fitted as a long loop to ensure folding-open of the airbag cover, as otherwise the forces acting on the hinge would be too great or the opening of the airbag too small, if otherwise the decorative surface itself would have to be deformed on folding-open of the airbag flap. As a result of this fitting of a loop, the cover is first accelerated, after tearing of the decorative surface above the airbag flap, into a translatory movement until the previously fitted loop is tensioned. Only in a further step is the cover forced, by the now tensioned hinge and the bearings of the connection of the hinge in the cover, into a rotatory movement. However, in this very short deflection point there are produced in a particularly short time of approximately 0.3 ms high forces which have to be transmitted directly to the plastics material via the yarns.
In this regard, it must also be borne in mind that the materials used are subject, specifically in the passenger compartment of a motor vehicle, partly owing to powerful solar irradiation, to particularly high changes in temperature. Specifications for temperature change tests carried out by car manufacturers anticipate temperatures in the range of from −35° C. to +85° C.
Obviously, however, the plastics material for the decorative carrier and in particular the decorative material does not have the same material properties over the entire temperature range. On the contrary, the plastics material is comparatively brittle at −35° C. and comparatively soft at +85° C. In the case of brittle plastics material, the yarns tend to become torn out of the plastics material bond; in the case of soft plastics material, on the other hand, the detachment takes place rather by way of extraction or peeling-away of the yarns from the plastics material carrier.